In surface-enhanced Raman scattering (SERS), vibrationally excitable levels of an analyte are probed. The energy of a photon shifts by an amount equal to that of the vibrational level (Raman scattering) excited by the photon. A Raman spectrum, which consists of a wavelength distribution of bands corresponding to molecular vibrations specific to the analyte being probed, may be detected to identify the analyte. In SERS, the analyte molecules are in close proximity, for instance, less than tens of nanometers, to metal nano-particles that may be or may not be coated with a dielectric, such as silicon dioxide, silicon nitride, and a polymer, that, once excited by light, set up plasmon modes, which create near fields around the metal nano-particles. These fields can couple to analyte molecules in the near field regions. As a result, concentration of the incident light occurs at close vicinity to the nano-particles, enhancing the Raman scattering from the analyte molecules.
SERS have recently been performed to probe fluids in vivo through implantation of the metal nano-particles subcutaneously. However, because fluids typically contain multiple types of species, some of the species that are not desired to be detected may bind onto the metal nano-particles or otherwise block the active sensing area. Permanent binding of the species of interest is also undesirable, as it limits the potential for continuous sensing. As a result conventional SERS devices are often unable to provide accurate measurements of desired species.